Amyotrophic lateral sclerosis (ALS) is a devastating neurodegenerative disease, which affects motor neurons of the spinal cord and cerebral cortex, leading to progressive paralysis and premature death. While the majority of the cases are sporadic and due to unknown causes, about 5-10% are familial (FALS), and of those approximately 25% are associated with mutations in the gene for Cu/Zn superoxide dismutase (SOD1). These mutations are the first identified cause of ALS, and their finding has allowed the development of a transgenic mouse model of the disease. Abnormal mitochondria and impaired mitochondrial respiratory chain function have been reported in motor neurons of patients with sporadic and ALS, as well as in the mouse models. A growing body of evidence suggests that impaired mitochondrial energy production and increased oxidative radical formation in the mitochondria and damage to the mitochondrial DNA (mtDNA) could be causally involved in motor neuron death in ALS. Indeed, several studies have postulated the involvement of mtDNA abnormalities in motor neuron degeneration. To define the role of mitochondrial alterations in the pathogenesis of ALS, we propose the following studies. 1) Analyze mitochondrial functions in brain and spinal cord in ALS transgenic mice at various stages of disease and in cultured cells expressing mutant human SOD1. Analyze the mtDNA from ALS transgenic mice in cybrids constructed with synaptosomes from brain and spinal cords. 2) Study the structural properties and the functional effects of SOD1 localized to mitochondria in transgenic animals and in cultured neuroblastoma cells expressing wild type and mutated h SOD1. 3) Analyze respiratory chain functions and mtDNA in individual motor neurons from ALS patients. We think that a better comprehension of the molecular basis of mitochondrial dysfunction in ALS will help us to understand the mechanisms responsible for the motor neuron loss and, in some cases may even identify the primary cause of the disease, with a potential impact on therapeutic strategies.